Microelectromechanical structure with enhanced rejection of acceleration noise

ABSTRACT

An integrated MEMS structure includes a driving assembly anchored to a substrate and actuated with a driving movement. A pair of sensing masses suspended above the substrate and coupled to the driving assembly via elastic elements is fixed in the driving movement and performs a movement along a first direction of detection, in response to an external stress. A coupling assembly couples the pair of sensing masses mechanically to couple the vibration modes. The coupling assembly is formed by a rigid element, which connects the sensing masses and has a point of constraint in an intermediate position between the sensing masses, and elastic coupling elements for coupling the rigid element to the sensing masses to present a first stiffness to a movement in phase-opposition and a second stiffness, greater than the first, to a movement in phase, of the sensing masses along the direction of detection.

BACKGROUND

1. Technical Field

The present disclosure relates to a microelectromechanical structurehaving enhanced mechanical characteristics for rejection of accelerationnoise, in particular, the following discussion will reference, withoutimplying any loss of generality, to a gyroscope of amicroelectromechanical type.

2. Description of the Related Art

Micromachining techniques enable manufacturing of microelectromechanicalstructures or systems (MEMS) within layers of semiconductor material,which have been deposited (for example, a layer of polycrystallinesilicon) or grown (for example, an epitaxial layer) on top ofsacrificial layers, which are removed via chemical etching. Inertialsensors, accelerometers, and gyroscopes made with this technology areexperiencing a growing success, for example, in the automotive field, inthe inertial-navigation sector, or in the sector of portable devices.

In particular, integrated gyroscopes made of semiconductor materialusing MEMS technology are known. These gyroscopes operate on the basisof the theorem of relative accelerations, exploiting the Coriolisacceleration. When an angular velocity is applied to a mobile mass thatis driven with a linear velocity, the mobile mass “feels” an apparentforce, called Coriolis' force, which determines a displacement thereofin a direction perpendicular to the direction of the linear velocity andto the axis about which the angular velocity is applied. The mobile massis supported via springs that enable a displacement thereof in thedirection of the apparent force. On the basis of Hooke's law, thedisplacement is proportional to the apparent force in such a way thatfrom the displacement of the mobile mass it is possible to detect theCoriolis' force and a value of the angular velocity that has generatedit. The displacement of the mobile mass can, for example, be detected ina capacitive way, determining, in conditions of resonance, thevariations of capacity caused by the movement of mobile electrodes,fixed with respect to the mobile mass and coupled to fixed electrodes.

MEMS gyroscopes generally have symmetrical sensing structures,comprising a pair of sensing masses for each axis of detection aboutwhich a corresponding angular velocity is detected. Ideally, analtogether symmetrical structure enables rejecting completely, by meansof the use of appropriate differential reading schemes, linear noiseaccelerations that are applied from the outside, for example, which canbe imputed to shock acting on the sensor or to the gravity acceleration.In fact, whereas the Coriolis' force tends to unbalance in oppositedirections, and substantially by the same amount, the sensing masses ofeach pair (generating movements “in phase-opposition”), the externalnoise accelerations determine displacements in the same direction andagain by the same amount (generating movements “in phase”). By executingthe difference of the electrical signals associated to the two sensingmasses of each pair, it is possible to measure the contribution due tothe Coriolis' force and reject completely the noise contributions of theaccelerations.

The inevitable spread of the manufacturing process, and in particularthe resulting differences, even minimal, in the mechanicalcharacteristics of the sensing masses and of the corresponding elasticsupporting elements, are such that gyroscopes of a traditional type arenot perfectly immune from acceleration noise coming from outside.

In fact, even though the vibration modes of the sensing masses areuncoupled and ideally at the same frequency, due to process spreads, theresonance frequencies of the two sensing masses of each pair cannot beperfectly coincident. For example, they can differ by 10-20 Hz, whichcauses, for high factors of merit Q, a poor rejection to the externalacceleration noise. In particular, external accelerations having afrequency close to the frequencies of resonance of the sensing massescan generate responses even considerably different in the two sensingmasses, thus generating a non-zero output from the corresponding readingelectronics (notwithstanding the differential scheme adopted is ideallyable to reject the noise). Considering that the resonance frequency ofthe sensing masses is usually comprised in the audio band (i.e., lessthan 20 kHz), it is evident that environmental noise can also generate,for the reason set forth above, even relevant noise at output.

BRIEF SUMMARY

The present disclosure provides an integrated MEMS structure thatincludes a driving assembly anchored to a substrate and configured to beactuated with a driving movement and a first sensing mass and a secondsensing mass suspended above the substrate and coupled to the drivingassembly via respective first elastic supports and configured to performa movement of detection along a first direction of detection, inresponse to an external stress. The MEMS structure includes a couplingassembly to couple mechanically the first sensing mass and the secondsensing mass to couple their vibration modes, that has a rigid elementbetween the first and the second sensing masses having a point ofconstraint in an intermediate position between the first and secondsensing masses and first and second elastic connectors to connectrespective ends of the rigid element to the first and second sensingmasses, the first and second elastic connectors and the rigid elementbeing configured to present a first stiffness to a movement inphase-opposition, and a second stiffness, greater than the firststiffness, to a movement in phase of the first and second sensing massesalong the first direction of detection.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, preferredembodiments thereof are now described, purely by way of non-limitingexample and with reference to the attached drawings, wherein:

FIG. 1 shows a schematic top plan view of a microelectromechanicalstructure of a MEMS gyroscope, of a known type;

FIGS. 2 a, 2 b show plots of electrical quantities corresponding to thegyroscope of FIG. 1;

FIG. 3 shows a schematic top plan view of a coupling structure betweensensing masses with movement of translation of a MEMS gyroscopeaccording to one aspect of the present disclosure;

FIG. 4 shows a side view of a coupling structure between sensing masseswith movement of rotation of a MEMS gyroscope according to a furtheraspect of the present disclosure;

FIGS. 5 a, 5 b show plots of electrical quantities corresponding to theMEMS gyroscope of FIG. 3 or FIG. 4;

FIG. 6 a shows a schematic top plan view of a uniaxial MEMS gyroscopeaccording to a first embodiment of the present disclosure;

FIG. 6 b shows a schematic top plan view of the MEMS gyroscope of FIG. 6a, and the deformation of corresponding elastic elements, duringdetection of a yaw angular velocity;

FIG. 7 a shows a schematic top plan view of a uniaxial MEMS gyroscope inaccordance with a second embodiment of the present disclosure;

FIG. 7 b shows a schematic top plan view of the MEMS gyroscope of FIG. 7a, and the deformation of corresponding elastic elements, during thedetection of a roll angular velocity;

FIG. 8 shows a schematic top plan view of a biaxial MEMS gyroscopeaccording to a third embodiment of the present disclosure, which is ableto detect angular velocities of yaw and roll;

FIG. 9 shows a schematic top plan view of a biaxial MEMS gyroscopeaccording to a fourth embodiment of the present disclosure, which isable to detect angular velocities of pitch and roll;

FIG. 10 shows a schematic top plan view of a triaxial MEMS gyroscopeaccording to a fifth embodiment of the present disclosure, which is ableto detect angular velocities of yaw, pitch, and roll; and

FIG. 11 shows a simplified block diagram of an electronic deviceprovided with a MEMS gyroscope according to yet a further aspect of thepresent disclosure.

DETAILED DESCRIPTION

Italian patent application No. TO2008A000981, filed by the presentApplicants on Dec. 23, 2008, describes a sensor with amicroelectromechanical integrated gyroscope with rotary driving movementand sensitive to yaw angular velocities, configured to reduce thesensitivity to external acceleration noise. FIG. 1 shows an exemplaryembodiment of a microelectromechanical gyroscope, designated by 100,made according to the teachings contained in the aforesaid patentapplication. The MEMS gyroscope 100, made starting from a die having asubstrate of semiconductor material (for example, silicon), comprises adriving mass 103 and a driving assembly 104.

The driving mass 103 has a substantially planar configuration with amain extension in a plane of the sensor xy (defined by a firsthorizontal axis x and a second horizontal axis y, orthogonal to oneanother, and substantially parallel to the plane of the substrate). Thedriving mass 103 is negligible in size, with respect to the mainextension, in a direction parallel to a vertical axis z, which formswith the first and the second horizontal axis x, y an orthogonal set ofaxes. The driving mass 103 defines centrally an empty space 106, acenter 105 of which coincides with a centroid and a center of symmetryof the entire sensor. The driving mass 103 is anchored to the substrateby anchor elements 107 a, set within the empty space 106, to which it isconnected through elastic anchorage elements 108 a.

The elastic anchorage elements 108 a enable a rotary movement of thedriving mass 103 about a driving axis passing through the center 105,parallel to the vertical axis z and perpendicular to the plane of thesensor xy, with a driving angular velocity {right arrow over (Ω)}_(a).

The driving assembly 104 comprises a plurality of groups of drivingelectrodes 109, extending outwards from the driving mass 103 in a radialdirection and set at the same angular distance apart, constituted byelectrodes in comb-fingered configuration. Appropriateelectrical-biasing signals coming from a driving circuit (not shown),determine, by means of the mutual and alternating attraction of theelectrodes, a oscillatory rotary motion of the driving mass 103 aboutthe driving axis, at a given oscillation frequency and at the drivingangular velocity {right arrow over (Ω)}_(a).

MEMS gyroscope 100 further comprises a pair of sensing masses 110 a, 110b, arranged within the empty space 106, for the detection of yaw angularvelocities {right arrow over (Ω)}_(i) acting about the vertical axis z.The sensing masses 110 a, 110 b are suspended with respect to thesubstrate and connected to the driving mass 103 via respective elasticsupporting elements 111, in such a way as to be fixed with respect tothe driving mass during its rotary driving motion, and to undergo alinear detection movement, substantially uncoupled with respect to thedriving movement, in a radial direction (coinciding with the secondhorizontal axis y), in response to the Coriolis' force. Coupled to eachof the sensing masses 110 a, 110 b are mobile electrodes 112, which formfirst and second detection capacitors with plane and parallel faces withrespective first and second fixed electrodes 113 a, 113 b, fixed withrespect to the driving mass 103.

During operation, the gyroscope 100 is able to detect the yaw angularvelocity {right arrow over (Ω)}_(i) acting about the vertical axis z. Inparticular, this angular velocity generates a Coriolis' force on thesensing masses 110 a, 110 b directed in a radial direction (directedhence as a centripetal force acting on the same masses), causing thedisplacement of the sensing masses, which move in phase-opposition inthe radial direction. In other words, the sensing masses move inopposite directions with respect to the radial direction and by the sameamount. The value of the resulting capacitive variation of thecorresponding detection capacitors is proportional to the angularvelocity {right arrow over (Ω)}_(i), which can hence be determined, in aknown manner, via a reading circuit, operating according to adifferential scheme. In particular, appropriate connections are madebetween the fixed electrodes 113 a, 113 b and mobile electrodes 112 insuch a way that the difference between electrical quantities correlatedto the variations of the first and second detection capacitors isamplified in a differential way.

In order to reduce the noise linked to external accelerations acting onthe sensing structure, which generate displacements in phase of thesensing masses 110 a, 110 bin the same radial direction (i.e.,displacements having the same value and oriented in the same direction),the MEMS gyroscope 100 further comprises a coupling structure designedto couple the sensing masses 110 a, 110 belastically to one another. Thecoupling structure comprises elastic coupling elements 115 a, 115 b,extending in the radial direction, each, starting from a respective oneof the sensing masses 110 a, 110 b, and connected to one another via aconnection element 116, set in a central position, for example, in aposition corresponding to the center 105. The connection element 116 isconfigured to have substantially negligible weight and dimensions, inparticular as compared with those of the sensing masses and of theelastic elements. The connection element 116 is also connected to thedriving mass 103 via further elastic supporting elements 18, whichextend in a direction transverse to the radial direction of the sensingmasses (along the first horizontal axis x).

The elastic coupling elements 115 a, 115 b have, during operation, thefunction of coupling the vibration modes of the sensing masses 110 a,110 b, and of giving rise to two different separate vibration modes ofthe resulting mechanical sensing structure. In particular, a firstvibration mode, in phase and a second vibration mode, inphase-opposition are generated having frequencies of resonance clearlyseparate from one another. In either case, the two sensing masses 110 a,110 b vibrate at the same frequency (irrespective of any possibleprocess spread). It is consequently easy, via the reading electronics,to reject the vibration mode in phase linked to the noise accelerations,and preserve, for the subsequent processing, just the vibration mode inphase-opposition representing the angular accelerations to be detected.

From the mechanical standpoint, the aforesaid two different vibrationmodes derive from the different modes of displacement of the sensingmasses 110 a, 110 b, during the movement in phase or inphase-opposition. In particular, during the movement inphase-opposition, the displacement of the sensing masses 110 a, 110barises both from the deformation of the elastic coupling elements 115a, 115 band from the deformation of the elastic supporting elements 111,so that the connection element 116 remains substantially stationary in acentral position. During the movement in phase, the elastic couplingelements 115 a, 115 bundergo a smaller deformation with respect to themotion in phase-opposition, moreover the further elastic supportingelements 18 (which in the motion in phase-opposition were substantiallystationary) undergo deformation, and the connection element 116 isdisplaced in the radial direction. There follows a stiffness and anatural frequency of the mode in phase that is sensibly smaller than thenatural frequency of the mode in phase-opposition (the stiffness of thestructure is hence as a whole reduced).

FIGS. 2 a and 2 b show results of a numeric processing, which sets indirect comparison the values obtained with the MEMS gyroscope 100 ofFIG. 1 (solid line) with the values obtained with a traditionalstructure of a known type (dashed line), without elastic couplingbetween the sensing masses. In particular, in this numeric processing aprocess spread has been simulated applying a difference of 1% in thestiffness of the elastic supporting elements associated to the sensingmasses, and a random noise of displacement has been applied to theconstraints of the same sensing masses (to simulate an externalacceleration noise). FIGS. 2 a and 2 b show, respectively in linear andlogarithmic scale, the signal at output from the reading electronics ofthe gyroscope (and hence the result of the operations of amplificationand demodulation, of a known type, of the electrical quantities atoutput from the detection capacitors).

From these plots, the presence of two distinct frequency peaks set at adistance of approximately 20 Hz may be noted, due to the differentresonance frequency of the two sensing masses. When they are notcoupled; it is also evident that an output from the reading electronicsthat is non-zero is present and has a significant value in the presenceof a noise acceleration (having values that can even be comparable withthe values assumed during detection of angular accelerations). Instead,the coupling of the vibration modes of the sensing masses 110 a, 110 bgenerates at output two contributions of noise at frequencies clearlyseparate from one another: one, corresponding to the vibration inphase-opposition, has a frequency approximately twice the other, for thevibration in phase. In addition, this coupling enables reduction of theoutput of the gyroscope, in response to an external noise acceleration,approximately 100 times than that obtained with a traditional solution(without coupling between the sensing masses).

The above embodiment finds immediate application for sensing masses fordetecting yaw angular velocities, in structures (possibly also insertedwithin multiaxial sensors) that envisage a motion of the sensing massesof translation in a plane along one and the same axis.

One embodiment of the present disclosure provides a mechanical couplingof sensing masses 10 a, 10 b of a MEMS gyroscope 1 in a sensor structureto couple their vibration modes and to reject the acceleration noise.The coupled sensing masses are moreover suited for application inbiaxial and triaxial sensor structures and for the detection of pitchand roll angular accelerations.

In detail, and with reference to the schematic representation of FIG. 3,a top plan view of a portion of the sensor structure, a driving movementis a translation in a direction of a first horizontal axis x of a planeof the sensor xy (coinciding with a plane of main extension of thestructure). The movement of detection executed by the sensing masses 10a, 10 b as a function of the resulting Coriolis' force occurs in adirection of a second horizontal axis y (as illustrated schematically bythe double-headed arrow).

The sensing masses 10 a, 10 b (constituting a pair of sensing masses fordetecting the yaw angular acceleration about a vertical axis z,orthogonal to the plane of the sensor xy) are connected to a drivingstructure 3 (here illustrated schematically) via respective elasticsupporting elements 11.

The sensing masses 10 a, 10 b are in this case mechanically coupled by acoupling structure 20, which comprises a rigid connection element 22.For example, a rod made of the same semiconductor material of which thesensing masses are made. The rigid connection element 22 having a firstend connected to a first sensing mass 10 a of the pair and a second end,opposite to the first, connected to a second sensing mass 10 b of thepair. The rigid connection element 22 also having at a portion thereofintermediate between the sensing masses, for example, a central portionthat is a point of constraint with respect to the movement of detectionand that is forced to remain substantially immobile with respect totranslation during the movement of the sensing masses.

The rigid connection element 22 extends (in this case, along the firsthorizontal axis x) in a direction transverse with respect to thedirection of the movement of detection of the sensing masses 10 a, 10 b.In addition, the rigid connection element 22 can be ideally consideredinfinitely rigid with respect to bending.

The coupling structure 20 includes a first coupling element 23, designedto constrain the aforesaid intermediate portion of the rigid connectionelement 22 to the substrate of the MEMS gyroscope 1 in a positioncorresponding to the aforesaid point of constraint. A second couplingelement 24, designed to connect elastically the first end of the rigidconnection element 22 to the first sensing mass 10 a. A third couplingelement 25, designed to connect elastically the second end of the rigidconnection element 22 to the second sensing mass 10 b.

In particular, the first, second, and third coupling elements 23, 24, 25are configured to hinge ideally the rigid connection element 22centrally to the substrate, and at its ends to the sensing masses 10 a,10 b. Each of the coupling elements consequently performingsubstantially the function of a hinge that is assumed ideally as nothaving any torsional stiffness and as not being compliant (i.e., havingan infinite stiffness) with respect to translation. The couplingelements 23, 24, 25 are configured so as to enable rotations, but nottranslations, of the rigid connection element 22 with respect to thepoint of constraint with the substrate or to the sensing masses.

During operation, the presence of the second and third coupling elements24, 25 at the end portions of the rigid connection element 22 enablesthe sensing masses 10 a, 10 b to oscillate in phase-opposition at afrequency determined uniquely by the elastic supporting elements 11 withwhich they are connected to the driving mass 3. In particular, thesensing masses 10 a, 10 b perform, during the motion inphase-opposition, displacements of the same amount and in oppositedirections along lines parallel to one another and to the secondhorizontal axis y, at a distance determined substantially by the lengthextension of the rigid connection element 22. During this motion inphase-opposition, the second and third elastic coupling elements 24, 25undergo elastic deformation, whilst the rigid connection element 22rotates rigidly in the plane of the sensor xy about its intermediatepoint of constraint.

In addition, the presence of the first coupling element 23 at thecentral portion of the rigid connection element 22 and of the associatedpoint of constraint to the substrate, substantially prevents themovement in phase of the sensing masses 10 a, 10 b, given thesubstantially infinite rigidity to the translation (in this case in thedirection of the second horizontal axis y) of the aforesaid firstcoupling element 23, and to the stiffness to bending of the same rigidconnection element 22. Basically, it is as if the natural frequency ofthe in-phase mode of oscillation tended to infinity, with the result ofdefining a mechanical system with a single degree of freedom and hencewith a single natural frequency (the useful one of the mode inphase-opposition).

The coupling structure 20 previously described applies in asubstantially similar manner in the case where the motion of the sensingmasses 10 a, 10 b, enabled by the elastic supporting elements 11, is nota translation, but a rotation out of the plane of the sensor xy. As willbe described in detail in what follows, this movement of detection isassociated to the detection of angular accelerations of pitch and/orroll about the first horizontal axis x and the second horizontal axis y,respectively of the plane of the sensor xy.

With reference to the schematic representation of FIG. 4, which showslaterally the sensing structure in a cross section through the plane ofthe sensor xy parallel to the substrate, the driving movement is atranslation in the direction of the first horizontal axis x of the planeof the sensor xy. The movement of detection of the sensing masses 10 a,10 b is caused by the resulting Coriolis' force and is in this caserepresented by rotations in phase-opposition (i.e., in the oppositedirection and by the same amount) of the sensing masses out of the planexy about an axis defined by the elastic supporting elements 11.

Also in this case, the movement in phase-opposition of the sensingmasses 10 a, 10 b is enabled by the rotation of the rigid connectionelement 22 about the point of constraint and out of the plane of thesensor xy. The movement in phase-opposition of the sensing mass is alsoenabled by the elastic deformation of the coupling elements 24, 25 andthe movement in phase of the same sensing masses is hindered by theimpossibility of the rigid connection element 22 of translating withrespect to the point of constraint, in this case in the direction of thevertical axis z, and by the stiffness to bending of the rigid connectionelement 22.

As will be described in detail hereinafter, it is also possible tocombine the two solutions illustrated so as to provide the coupling ofthe various pairs of sensing masses in the case of biaxial sensors (withdetection of the pairs of angular velocities of pitch and roll, pitchand yaw, or roll and yaw) or in the case of triaxial sensors (withdetection of the angular velocities of pitch, roll, and yaw).

The coupling structure 20 is obtained using elements that have not, ofcourse, infinite stiffness. In addition, the torsional stiffness of thecoupling elements 23, 24, 25 cannot be zero. Consequently, the spuriousfrequency (associated to the motion in phase) does not tend to infinity,but to a high, but finite value. In addition, the presence of thecoupling structure 20 leads also to an increase in the frequency of theuseful vibration mode (motion in phase-opposition), with respect to thecase with uncoupled masses. With an adequate sizing of the couplingstructure 20 it is possible in any case, with a given natural frequencyof the useful mode, to reach very high values of spurious frequency andthus obtain a good separation of the vibration modes.

For example, FIGS. 5 a, 5 b show the result of a numeric processingsimilar to the one previously described with reference to FIGS. 2 a, 2b, setting directly in comparison the values obtained with the couplingstructure 20 (solid line) with the values obtained in a structure withuncoupled sensing masses (in dashed). From the plots an evident decreaseof the sensitivity of the gyroscope to the external acceleration noisemay be noted, and moreover the presence (in the shown frequency band) ofa single peak (equal to approximately 400 Hz), corresponding to thenatural frequency of the motion in phase-opposition (signal useful fordetection). The spurious frequency of the motion in phase is in factdisplaced to far higher frequencies (higher than the frequency bandhighlighted, i.e., higher than 1600 Hz).

Some examples of layout of uniaxial, biaxial, or triaxialmicroelectromechanical gyroscopes will now be illustrated, implementingthe coupling structure 20 between the corresponding sensing masses.

In particular, in these examples, in order to implement each of thecoupling elements 23, 24, 25 (having functions of a hinge at therespective hinge point) the solution is adopted consisting in the use oftwo springs having a rectilinear extension set, one as a prolongation ofthe other in a direction transverse to the direction of extension of therigid coupling element 22. Starting from the respective hinge point, soas to operate in bending-in-the-plane (the plane of the sensor) ortorsion during the motion in phase-opposition of the sensing masses, andto operate in tension/compression or bending-out-of-the-plane (again,the plane of the sensor) during the motion in phase. The behavior ofthis pair of flexible elements approximates very well that of a hingepositioned in the point of contact of the ends in common of therectilinear springs, if the rectilinear springs have a stiffness withrespect to the deformations oftension/compression/bending-out-of-the-plane much greater than thestiffness with respect to the deformations ofbending-in-the-plane/torsion.

FIG. 6 a shows a first embodiment of a MEMS gyroscope 30, of a uniaxialtype, sensitive to yaw angular velocities. In this figure, as in thesubsequent ones, similar reference numbers are used to designateelements similar to other ones already described.

The MEMS gyroscope 30 has a structure symmetrical with respect to thesecond horizontal axis y, comprising a first, substantially C-shaped,driving mass 103 a and a second driving mass 3 b substantially shapedlike a reversed C, arranged facing one another and defining internally arespective empty space 6 a, 6 b.

The driving masses 3 a, 3 b are anchored to the substrate (notillustrated herein) of the semiconductor-material die in which the MEMSgyroscope 30 is made, by anchorage means 7 a set externally to therespective empty space 6 a, 6 b, to which they are connected by means ofelastic anchorage elements 8 a extending along the first horizontal axisx. The driving masses 3 a, 3 b are actuated by respective sets ofdriving electrodes 9 (in comb-fingered configuration) in such a way asto generate a driving movement of translation in the direction of thefirst horizontal axis x.

The MEMS gyroscope 30 further comprises the first sensing mass 10 a andthe second sensing mass 10 b, each arranged in the respective emptyspace 6 a, 6 b inside the respective driving mass 3 a, 3 b. The firstand second sensing masses 10 a, 10 b are connected by means of elasticsupporting elements 11 extending along the second horizontal axis y. Theelastic supporting elements 11 are configured in such a way that thesensing masses 10 a, 10 b are dragged by the respective driving mass 3a, 3 b during the driving movement, and also perform a movement ofdetection, uncoupled from the driving movement, in particular a movementof translation along lines parallel to one another and to the secondhorizontal axis y. The sensing masses 10 a, 10 b have a substantiallyrectangular shape, elongated in a direction of the second horizontalaxis y, and are connected to respective mobile electrodes 12, extendinglaterally from them along the first horizontal axis x. The mobileelectrodes 12 form detection capacitors with plane and parallel faceswith respective first and second fixed electrodes 13 a, 13 b, anchored(in a way not illustrated) to the substrate, so as to be immobile withrespect to the movement of detection of the sensing masses 10 a, 10 b.

The rigid connection element 22 of the coupling structure 20 extendsbetween the first and the second sensing mass 10 a, 10 b along the firsthorizontal axis x, in a direction transverse to the direction of theaforesaid movement of detection, with the central portion thereof hingedto the substrate. In particular, the central portion is connected to afirst constraint anchorage 32 a by means of a first rectilinear spring34 a, extending along the second horizontal axis y. The central portionis also connected to a second constraint anchorage 32 b by means of asecond rectilinear spring 34 b, which also extends along the secondhorizontal axis y, as a prolongation of the first rectilinear spring 34a, starting from the aforesaid central portion. As will be illustratedhereinafter, the constraint anchorages 32 a, 32 b are pillars made ofsemiconductor material extending from the substrate as far as the planeof the sensor xy.

The first and second rectilinear spring 34 a, 34 b form the firstcoupling element 23 of the coupling structure 20, with the function tohinge the rigid connection element 22 to the substrate (the rectilinearsprings 34 a, 34 b have in fact a stiffness to tension/compression muchgreater than the stiffness with respect to bending-in-the-plane).

The second and third coupling elements 24, 25 are also constituted by apair of rectilinear springs 37 a, 37 b extending along the secondhorizontal axis y as a prolongation of one another starting from, and onopposite sides of, the rigid connection element 22. The ends of therectilinear springs 37 a, 37 b that are not in common are connected tocorresponding end portions of the respective sensing masses 10 a, 10 bby connecting portions 35, transverse to the springs 37 a, 37 b andextending along the first horizontal axis x.

During operation, as illustrated in FIG. 6 b, due to the drivingmovement of the driving masses 3 a, 3 b along the first horizontal axisx, and in the presence of a yaw angular velocity to be detected, aCoriolis' force is generated on the corresponding sensing masses 10 a,10 b, directed in opposite directions (as highlighted by the arrows)along the second horizontal axis y. In the resulting motion inphase-opposition, the sensing masses 10 a, 10 b move by the same amountin opposite directions along the second horizontal axis y, causingdeformation in torsion/bending of the first, second, and third couplingelements 23, 24, 25, and the rotation of the rigid connection element 22in the plane of the sensor xy about the hinge point/constraint (inparticular, about an axis parallel to the vertical axis z and passingthrough said hinge point/constraint).

Instead, an external acceleration noise, which would tend to move thesensing masses 10 a, 10 b by the same amount and in the same directionof the second horizontal axis y, is in actual fact countered by thepresence of the constraint to the substrate of the rigid connectionelement 22, which is constrained, with respect to the translation in theplane of the sensor xy along the same second horizontal axis y, by thepresence of the respective rectilinear springs 37 a, 37 b (in otherwords, being substantially immobile with respect to this translation).

FIG. 7 a shows a second embodiment of the present disclosure,corresponding to a MEMS gyroscope 40, which is able to detect rollangular velocities, having sensing masses that rotate out of the planeof the sensor xy (moving in the direction of the vertical axis z, if thesmall oscillations are considered).

This embodiment differs from the previous one, for a differentconfiguration of the sensing masses 10 a, 10 b that rotate out of theplane of the sensor xy about the axis defined by the elastic supportingelements 11, herein constituted by rectilinear elements extending alongthe second horizontal axis y coupled to an end portion of the respectivesensing mass 10 a, 10 b, on opposite sides with respect to the firsthorizontal axis x. A respective fixed electrode, herein designated by 13a, is set underneath each sensing mass 10 a, 10 b and on top of thesubstrate; the same sensing masses 10 a, 10 b constitute herein themobile electrode facing the fixed electrode 13 a. The sensing masses 10a, 10 b hence extend in cantilever fashion above the respective fixedelectrode 13 a and the substrate, starting from the respective elasticsupporting elements 11.

During operation, due to the driving movement of the driving masses 3 a,3 b along the first horizontal axis x and in the presence of a rollangular velocity about the second horizontal axis y, a Coriolis' forceis generated on the corresponding sensing masses 10 a, 10 b, oriented inthe direction of the vertical axis z.

As illustrated in FIG. 7 b (which shows the deformation of the structureduring the motion in phase-opposition), the sensing masses 10 a, 10 brotate out of the plane of the sensor xy, by the same amount and inopposite directions (moving away from, or approaching, the substrate,herein designated by the reference number 36, and the respective fixedelectrode 13 a). Again, this movement of detection is enabled by thedeformation in torsion of the first, second, and third coupling elements23, 24, 25. During this motion in phase-opposition, the rotation of therigid connection element 22 out of the plane xy also occurs, about thepoint of constraint (in this case, about an axis parallel to the secondhorizontal axis y extending along the rectilinear springs 34 a, 34 b).In FIG. 7 b, it is also illustrated the constraint anchorage 32 b,coupled to the substrate 36, shaped like a pillar extending verticallystarting from the substrate 36 as far as the plane of the sensor xy, andat which the rectilinear springs 34 a, 34 b are arranged.

Instead, noise accelerations do not ideally produce any displacement ofthe sensing masses 10 a, 10 b; the displacement of translation in thevertical direction z of the rigid connection element 22 is in facthindered by the stiffness to bending-out-of-the-plane of the firstcoupling element 23, which constrains the rigid connection element 22 tothe substrate 36, as well as by the stiffness to bending of the samerigid connection element 22.

FIG. 8 shows a third embodiment of the present disclosure, correspondingto a MEMS gyroscope 60 of a biaxial type, which is able to detect in asubstantially uncoupled way, angular velocities about an axis of yaw andan axis of roll, combining the sensing structures previously describedindividually.

For this purpose, within the empty space 6 a, 6 b of each driving mass 3a, 3 b, two sensing masses are present (so as to form two pairs ofsensing masses, one pair for each axis of detection), one arranged so asto translate in a direction in the plane of the sensor xy (inparticular, along the second horizontal axis y), and the other arrangedso as to rotate out of the same plane of the sensor xy.

In detail, sensing masses 10 c, 10 d of a second pair (designed fordetection of yaw angular velocities) are connected directly to therespective driving mass 3 a, 3 b by means of the elastic supportingelements 11, and are shaped like a C (or like a reversed C), defininginternally a further empty space 38 a, 38 b. Associated to each of thesesensing masses 10 c, 10 d are mobile electrodes 12, comb-fingered tofixed electrodes 13 a, 13 b, in a substantially similar manner as whatdescribed previously.

The sensing masses 10 a, 10 b of the first pair (designed for thedetection of roll angular velocities) are each arranged in therespective empty space 38 a, 38 b defined by a corresponding sensingmass 10 c, 10 d of the second pair, and are connected thereto by meansof further elastic supporting elements 39. The sensing masses 10 a, 10 bare hence connected to the respective driving mass 3 a, 3 b via theinterposition of a corresponding sensing mass 10 c, 10 d of the secondpair (an appropriate rigidity of the further elastic supporting elements39, with respect to the driving motion, is provided for this purpose).As described previously, the sensing masses 10 a, 10 b of the first pairextend in cantilever fashion from the respective elastic supportingelements 39, and face respective fixed electrodes 13 a set on top of thesubstrate 36 (not shown).

The coupling structure 20, made in a way substantially similar to whatwas described previously, is in this case connected directly to thesensing masses 10 a, 10 b of the first pair, and, via these, indirectlyto the sensing masses 10 c, 10 d of the second pair (and there is forthis purpose provided an appropriate rigidity of the further elasticsupporting elements 39 to the motion of translation of the sensingmasses 10 c, 10 d of the second pair).

During operation, a yaw angular velocity determines a Coriolis' forceoriented in the direction of the second horizontal axis y and aconsequent movement of translation of the sensing masses 10 c, 10 d ofthe second pair, which draw along rigidly in said movement also thesensing masses 10 a, 10 b of the first pair (this movement is enabled bythe deformation in bending-in-the-plane of the elastic elements of thecoupling structure 20). In a similar way, a roll angular velocitydetermines a Coriolis' force oriented in the direction of the verticalaxis z and a consequent movement of rotation out of the plane of thesensor xy of the sensing masses 10 a, 10 b of the first pair (thismovement is again enabled by the deformation, in torsion, of the elasticelements of the coupling structure 20). External noise accelerations,for reasons similar to what has been illustrated previously, do notdetermine, instead, appreciable displacements of the sensing masses 110a, 110 b and 110 c, 110 d of each pair, given the presence of theconstraint to the substrate 36 of the rigid connection element 22, in anintermediate position between the sensing masses, and to the rigidity ofthe same rigid connection element 22.

A fourth embodiment of the present disclosure, illustrated in FIG. 9,implements a MEMS biaxial gyroscope 70, sensitive to angular velocitiesabout the axis of roll and the axis of pitch.

Also in this case, two pairs of sensing masses 10 a, 10 b and 10 c, 10 dare hence present (one pair of masses for each axis of detection), allarranged (in a manner corresponding to what was described previously) soas to be able to rotate out of the plane of the sensor xy, aboutrespective elastic supporting elements 11 connected to correspondingdriving masses 3 a, 3 b and 3 c, 3 d (in this case, four in number, onefor each sensing mass). The MEMS gyroscope 70 has a resulting structurethat is symmetrical both with respect to the first horizontal axis x andwith respect to the second horizontal axis y; the pair of sensing masses10 a, 10 b and driving masses 3 a, 3 b corresponding to the roll axisare arranged aligned along the first horizontal axis x, whilst the pairof sensing masses 10 c, 10 d and of driving masses 3 c, 3 dcorresponding to the pitch axis are arranged aligned along the secondhorizontal axis y, with an overall crosswise arrangement of the variousmasses.

For coupling together the sensing masses of the two pairs, it is in thiscase provided a different configuration of the coupling structure 20,set at the center of the aforesaid cross defined by the sensing anddriving masses.

In particular, the rigid connection element 22 comprises herein a firstframe element 22 a, having side portions parallel in pairs to thehorizontal axes x and y, and a second frame element 22 b, set internallywith respect to the first frame element 22 a (parallel thereto). Theside portions both of the first frame element 22 a and of the secondframe element 22 b are constituted by rigid elements (for example, beamsmade of semiconductor material). The side portions parallel to thesecond horizontal axis y of the first frame element 22 a are connectedto corresponding side portions (again parallel to the second horizontalaxis y) of the second frame element 22 b by means of substantiallyelastic connection elements 50, directed along the first horizontal axisx; moreover, the side portions parallel to the first horizontal axis xof the second frame element 22 b (set more internally) are hinged to thesubstrate 36 by means of a single constraint anchorage, hereindesignated by 32 a, to which they are connected by means of respectiverectilinear springs 34 a, 34 b (parallel to the second horizontal axisy), constituting the first coupling element 23. The side portions of thefirst frame element 22 a (set more externally) are moreover coupled torespective sensing masses 10 a-10 d by means of respective second andthird coupling elements 24, 25 (for a total of four coupling elements,one for each sensing mass), to which they are connected by means offurther connection elements 52, also substantially elastic and directedalong the first or second horizontal axis x, y.

During operation, the rotation movements in phase-opposition of thesensing masses 10 a-10 b, 10 c-10 d of the two pairs are transmitted tothe first frame element 22 a by the second and third coupling elements24, 25, and, via the connection elements 50, are transmitted to thesecond frame element 22 b, with an associated deformation of therectilinear springs 34 a, 34 b of the first coupling element 23.Instead, the rotation movements in phase of the same sensing masses, dueto noise accelerations, do not cause appreciable displacements, giventhe rigidity to translation of the entire coupling structure 20 (and, inparticular, of the rectilinear springs 34 a, 34 b associated to theconstraint anchorage 32 a).

A fifth embodiment of the present disclosure, illustrated in FIG. 10,uses a coupling structure 20 substantially similar to the oneillustrated in FIG. 9 and sensing structures similar to the onesdescribed previously, to provide a triaxial MEMS gyroscope 80, sensitiveto angular velocities about the axis of yaw, as well as the axes of rolland pitch.

In this case, four driving masses 3 a-3 d and three pairs of sensingmasses (10 a-10 b, 10 c-10 d and 10 e-10 f) are present, eachcorresponding to a respective axis of detection of the MEMS gyroscope80. In particular, with respect to the solution illustrated in FIG. 9,there is a further pair of C-shaped sensing masses 10 e-10 f, arrangedexternally to the sensing masses 10 c-10 d corresponding to the axis ofpitch, and elastically connected to them substantially as described withreference to the third embodiment of FIG. 8 (equivalently, the thirdpair of sensing masses could be set along the first horizontal axis x,externally to the sensing masses 10 a, 10 b corresponding to the axis ofroll).

Applying the principles highlighted previously, it is immediatelypossible to verify that this structure enables detection of themovements in phase-opposition of the sensing masses corresponding to theangular velocities of yaw, roll or pitch, substantially preventing themovements in phase of the same sensing masses (and thus enablingrejection of the noise accelerations).

The advantages of the microelectromechanical gyroscope made according tothe present disclosure are clear from the foregoing description.

In particular, it is again emphasized that the particular solution formechanical coupling of the sensing masses for sensing the angularvelocities enables rejection of the external acceleration noise (forexample, due to environmental noise or other form of noise), also in thepresence of manufacturing process spreads.

The present coupling solution can be used for coupling sensing massescorresponding to a number of axes (yaw, roll, pitch) simultaneously; thepresent coupling solution is in fact advantageously applicable also inbiaxial or triaxial gyroscopes, enabling in fact the integration withsensing structures for sensing pitch and/or roll angular accelerations.

This solution also envisages a separation of the resonance frequenciesof detection such as to displace the undesired vibration mode (i.e., theone in which the sensing masses oscillate in phase) to a higherfrequency with respect to the vibration mode useful for detection(oscillation in phase-opposition), the two frequencies being in any casevery distant from one another (the greater the separation, the betterthe effects of rejection to the noise).

Thanks to this feature, the spurious oscillation that is obtained atoutput from the gyroscope is very small and, being at a frequency veryfar from the frequency of interest, can be effectively filtered by thereading electronics.

In particular, separating the resonance frequency of the mode in phaseby bringing it to high frequency is even more advantageous, because, ingeneral, the higher the frequency, the smaller is the amount of externalaccelerations acting on the sensor. In addition, bringing the mode inphase at a higher frequency it is possible to obtain a greaterseparation of the vibration modes (given that, while stiffening of thesensing structure is always advantageous, a reduction of its stiffnesscan never be excessive); filtering of the frequency contributions of theundesirable mode is more effective.

A further advantage of the present coupling structure is that, in thecase of capacitive reading, the stiffness increase for the vibration ofthe in-phase mode leads to a smaller variation of gap between theelectrodes (and hence of the device sensitivity) in the presence of theacceleration of gravity.

In addition, the coupling of the sensing masses increases considerably(substantially doubles, given the same frequency) the stiffness of thesystem in the direction of detection, rendering it more robust, forexample, creating a greater resistance to the problem of stiction(sticking of the mobile electrodes against the fixed elements of thestructure). In a known way, stiction is a phenomenon due to the force ofadhesion between surfaces in contact. If following upon a shock, themobile elements come into contact with the fixed elements, between thesurfaces of contact a force of adhesion is generated, which tends tokeep them united. Opposed to this force of adhesion is the force ofelastic return, which is proportional to the stiffness of the system.Given the same frequency, the greater the mass involved the greater thestrength. In the uncoupled system the mass involved is given by the massof the single sensing mass. In the coupled system, instead, the massinvolved is the total mass of the two sensing masses. If the masses areequal, the strength involved then doubles. Intuitively, in the coupledsystem, if a sensing mass is subjected to the stiction phenomenon, notonly its elastic elements, but also those of the second sensing massconnected thereto oppose the force of adhesion.

Basically, the aforesaid features render the MEMS gyroscopesparticularly indicated for integration in an electronic device 50, asillustrated in FIG. 11, that can be used in a plurality of electronicsystems, for example, in inertial-navigation systems, in automotivesystems, or in systems of a portable type, such as for example a PDA(Personal Digital Assistant), a portable computer, a mobile phone, adigital audio player, a photographic or video camera, the electronicdevice 50 generally being able to process, store, transmit and receivesignals and information.

The electronic device 50 comprises: a driving circuit 51, operativelycoupled to the driving assembly to impart the driving movement on thevarious driving masses 3, and supply biasing signals to themicroelectromechanical structures. A reading circuit 52, operativelycoupled to the detection capacitors of the sensing masses to detect theamount of the displacements of the same sensing masses and hencedetermine the angular velocities acting on the structure; and anelectronic control unit 54, for example, a microprocessor control unit,connected to the reading circuit 52, and designed to supervise theoverall operation of the electronic device 50, for example, based on theangular velocities detected and determined.

Finally, it is clear that modifications and variations can be made towhat has been described and illustrated herein, without therebydeparting from the scope of the present disclosure.

In particular, it is clear that the solution described for implementingthe coupling elements (as mentioned previously, having a hinge function)of the coupling structure between the sensing masses of each pair is notto be understood as in any way limiting, and that further solutions canbe equally envisaged for this purpose.

For example, the opposed springs, set as a prolongation of one anotherto form these coupling elements, could be of the folded type, having inany case a main extension (or development) in the same direction (inparticular, in the direction transverse to the rigid element 22).

In addition, the intermediate point of constraint of the rigidconnection element 22 could be connected to a different element, fixedwith respect to the movement of detection, for example to the drivingmass 3 instead of to the substrate 36.

In general, it is clear that modifications to the configuration of someof the structural elements of the MEMS gyroscopes may be envisaged. Forinstance, the driving masses 3 may have a different shape, i.e.,different from the one illustrated, just as the shape of the sensingmasses may also be different.

In addition, the displacement of the sensing masses can be determinedwith techniques different from the capacitive one, for example by meansof the detection of a magnetic force; and the driving movement can begenerated in a different way, for example by means of parallel-plateelectrodes, or else with a magnetic actuation. The various embodimentsdescribed above can be combined to provide further embodiments. All ofthe U.S. patents, U.S. patent application publications, U.S. patentapplication, foreign patents, foreign patent application and non-patentpublications referred to in this specification and/or listed in theApplication Data Sheet are incorporated herein by reference, in theirentirety. Aspects of the embodiments can be modified, if necessary toemploy concepts of the various patents, application and publications toprovide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A structure, comprising: a substrate; aplurality of driving electrodes anchored to the substrate; a firstanchorage formed on the substrate; a second anchorage formed on thesubstrate; a first driving mass coupled to the first anchorage andconfigured to be driven by at least some of the plurality of drivingelectrodes; a second driving mass coupled to the second anchorage andconfigured to be driven by at least some of the plurality of drivingelectrodes; a first sensing mass elastically coupled to the firstdriving mass, the first sensing mass having a first surface; a secondsensing mass elastically coupled to the second driving mass, the secondsensing mass having a second surface facing the first surface; and arigid element coupled between the first and second sensing masses, therigid element having a longitudinal extension that is transverse to thefirst and second surfaces.
 2. The structure of claim 1, wherein thefirst sensing mass is elastically coupled to a first interior surface ofthe first driving mass and the second sensing mass is coupled to secondinterior surface of the second driving mass.
 3. The structure of claim2, wherein the first sensing mass includes an electrode extending fromthe first sensing mass towards the first interior surface of the firstdriving mass and the second sensing mass includes an electrode extendingfrom the second sensing mass towards the second interior surface of thesecond driving mass.
 4. The structure of claim 3, wherein a first set offixed electrodes is positioned on the substrate between the firstsensing mass and the first driving mass and a second set of fixedelectrodes is positioned on the substrate between the second sensingmass and the second driving mass, the first set of fixed electrodesbeing configured to interact with the electrode extending from the firstsensing mass and the second set of fixed electrodes being configured tointeract with the electrode extending from the second sensing mass. 5.The structure of claim 1, wherein the first sensing mass includeselastic connection elements that couple a first spring to both a firstend and a second end of the first sensing mass and the second sensingmass includes elastic connection elements that couple a second springcoupled to both a first end and a second end of the second sensing mass,the rigid element being coupled to the first spring and the secondspring.
 6. The structure of claim 5, wherein the longitudinal extensionof the rigid element extends along an axis that passes through a centrallocation of both the first and second sensing masses.
 7. The structureof claim 6, wherein the first sensing mass includes an electrodeextending along the axis away from the second sensing mass and thesecond sensing mass includes an electrode extending along the axis awayfrom the first sensing mass.
 8. The structure of claim 6, furthercomprising a third spring and a fourth spring positioned between thefirst and second sensing masses, the third spring coupled between afirst anchor and the rigid element and the fourth spring coupled betweena second anchor and the rigid element, the third and fourth springsextending transverse to the rigid element.
 9. The structure of claim 8,wherein the rigid element is longer than a minimum distance between thefirst driving mass and the second driving mass.
 10. The structure ofclaim 1, wherein the first driving mass and the second driving mass havea non-rectangular shape.
 11. The structure of claim 10, wherein theshape of the first driving mass is a mirror image of the second drivingmass.
 12. The structure of claim 10, wherein the first driving mass andthe second driving mass are both C-shaped.
 13. The structure of claim 1,wherein the plurality of driving electrodes includes a first set and asecond set of the plurality of driving electrodes, the first set beingelectrically coupled to the first driving mass and the second set beingelectrically coupled to the second driving mass.
 14. A system,comprising: a driving circuit; a reading circuit; a structure coupled tothe driving circuit and to the reading circuit, the structure including:a substrate; a plurality of driving electrodes anchored to the substrateand coupled to the driving circuit; a first driving mass suspended abovethe substrate and electrically coupled to at least some of the pluralityof driving electrodes; a second driving mass suspended above thesubstrate and electrically coupled to at least some of the plurality ofdriving electrodes; a first sensing mass elastically coupled to thefirst driving mass, the first sensing mass having a first surface; asecond sensing mass elastically coupled to the second driving mass, thesecond sensing mass having a second surface facing the first surface;and a rigid element coupled between the first and second sensing masses,the a rigid element extending transversely between the first and secondsurfaces.
 15. The system of claim 14, wherein the first sensing mass iselastically coupled to an interior surface of the first driving mass andthe second sensing mass is coupled to an interior surface of the seconddriving mass.
 16. The system of claim 15, wherein the first sensing massincludes an electrode extending from the first sensing mass towards theinterior surface of the first driving mass and the second sensing massincludes an electrode extending from the second sensing mass towards theinterior surface of the second driving mass.
 17. The system of claim 16,wherein a first set of fixed electrodes is positioned on the substratebetween the first sensing mass and the first driving mass and a secondset of fixed electrodes is positioned on the substrate between thesecond sensing mass and the second driving mass, the first set of fixedelectrodes being configured to interact with the electrode extendingfrom the first sensing mass and the second set of fixed electrodes beingconfigured to interact with the electrode extending from the secondsensing mass.
 18. The system of claim 14, wherein the first sensing massincludes elastic connection elements that couple a first spring to botha first end and a second end of the first sensing mass by and the secondsensing mass includes elastic connection elements that couple a secondspring to both a first end and a second end of the second sensing mass,the rigid element being coupled to the first spring and the secondspring.
 19. A method, comprising: forming a structure integrated with asubstrate of a die, the forming of the structure including: coupling afirst driving mass to the substrate; coupling a second driving mass tothe substrate adjacent to the first driving mass; coupling a drivingassembly to the first driving mass and to the second driving mass;elastically coupling a first sensing mass to the first driving mass, thefirst sensing mass having a first surface; elastically coupling a secondsensing mass to the second driving mass, the second sensing mass havinga second surface facing the first surface; and coupling a rigid elementbetween the first and second sensing masses, the rigid element extendingtransversely to the first and second surfaces.
 20. The method of claim19, further comprising elastically coupling the first sensing mass to aninterior surface of the first driving mass and elastically coupling thesecond sensing mass to an interior surface of the second driving mass.21. The method of claim 20, further comprising forming an electrodeextending from the first sensing mass towards the interior surface ofthe first driving mass and forming an electrode extending from thesecond sensing mass towards the interior surface of the second drivingmass.
 22. The method of claim 21, further comprising forming a first setof fixed electrodes positioned on the substrate between the firstsensing mass and the first driving mass and forming a second set offixed electrodes positioned on the substrate between the second sensingmass and the second driving mass, the first set of fixed electrodesbeing configured to interact with the electrode extending from the firstsensing mass and the second set of fixed electrodes being configured tointeract with the electrode extending from the second sensing mass. 23.The method of claim 22, wherein the rigid element extends along an axisbetween a central location of the both the first and second sensingmasses.
 24. The method of claim 23, wherein the electrode from the firstsensing mass extends along the axis away from the second sensing massand the electrode from the second sensing mass extends along the axisaway from the first sensing mass.